1. Field of the Invention
This invention relates to a control system for internal combustion engines, which controls the supply of fuel injected into an intake pipe of the engine in a manner compensating for a fuel amount adhering to the inner surface of the intake pipe.
2. Prior Art
In conventional internal combustion engines of the type that fuel is injected into an intake pipe, there is a problem that some of injected fuel adheres to the inner surface of the intake pipe, so that a required amount of fuel cannot be drawn into the combustion chamber. To solve this problem, there has been proposed a fuel supply control method (adherence correction) by Japanese Provisional Patent Publication (Kokai) No. 61-126337, which estimates a fuel amount which is to adhere to the inner surface of the intake pipe and one which is to be drawn into the combustion chamber by evaporation from the fuel adhering to the intake pipe, and determines a fuel injection amount in dependence on the estimated fuel amounts.
Further, conventionally internal combustion engines are known, for example, from Japanese Patent Publication (Kokoku) No. 2-50285, in which operating characteristics of intake valves and exhaust valves of the engine, i.e. valve timing (valve opening/closing timing and/or valve lift) are changeable.
Furthermore, to apply the above-mentioned control method to the above-mentioned type internal combustion engines, a method has been already proposed by the present assignee by Japanese Provisional Patent Publication (Kokai) No. 5-99030 and U.S. Pat. No. 5,215,061 corresponding thereto, which can accurately control the air-fuel ratio of a mixture supplied to the engine by correcting an adherent fuel amount and a carried-off fuel amount in accordance with operating characteristics of intake valves and/or exhaust valves of the engine.
According to the above proposed adherence-correcting method, the adherent fuel amount and the carried-off fuel amount are calculated by the use of a direct supply ratio A and a carry-off ratio B in the following manner: The direct supply ratio A is defined as a ratio of a fuel amount directly or immediately drawn into a combustion chamber in an operating cycle of the engine to the whole fuel amount injected in the same operating cycle, and the carry-off ratio B is defined as a ratio of a fuel amount carried off the inner surface of the intake pipe by evaporation, etc. and drawn into the combustion chamber in the present operating cycle to the whole fuel amount which adhered to the inner surface of the intake pipe in the last or immediately preceding operating cycle. The adherent fuel amount is estimated based on the direct supply ratio A and the carry-off ratio B, and the carried-off fuel amount is estimated based on the carry-off ratio B and the above estimated adherent fuel amount. The direct supply ratio A and the carry-off ratio B are determined based on a plurality of parameters which are closely related to the adherence correction, such as engine coolant temperature, engine rotational speed, and intake pipe absolute pressure.
In the above adherence-correcting method, however, the direct supply ratio A and the carry-off ratio B are parameters calculated independently of each other, and the adherence correction of the fuel injection amount is carried out without taking into account the relationship between the direct supply ratio A and the carry-off ratio B. As a result, an inconvenience can sometimes occur depending on the above relationship.
For example, when the relationship of A&lt;B stands, the fuel injection amount after the adherence correction converges to a desired value while fluctuating. Consequently, the air-fuel ratio of a mixture supplied to the engine can deviate from a desired air-fuel ratio due to the fluctuation of the fuel injection amount, resulting in degraded exhaust emission characteristics and degraded drivability of the engine.